Many natural and synthetic compositions may benefit from additives that modify rheology. For example, lubricant oil formulations generally contain viscosity index improvers derived from polyolefins that modify rheological behavior. There have been many attempts to develop polyolefin additives that have a high thickening efficiency without raising the average ethylene content or the propensity to chain scission under shear.
Many conventional polyolefin additives, however, suffer from unfavorable characteristics such as: (a) a high molecular weight fraction such that they are more affected by shear induced degradation of the molecular weight—such compositions have an unfavorable thickening efficiency (TE)/shear stability index (SSI) ratio in that they have a lower thickening efficiency for a given SSI; (b) preparation with conventional catalysts, which contain both a broad polydispersity index and a broad compositional distribution. Blends of amorphous and semi crystalline polyolefins have a significant and predetermined broadening of the polydispersity index and intermolecular compositional heterogeneity.
It would be desirable to produce polyolefin additives that have very little crystallinity or are completely amorphous to minimize interactions with wax molecules in the base oils. Such polymers, however, have low bulk viscosities and are tacky and extremely difficult to handle in typical finishing operations.
Accordingly, there remains a need for viscosity index improving compositions that promote the following in lubricant oils, while having a low ethylene content and maintaining adequate pellet handling characteristics: (a) a more constant viscosity over a broad range of temperatures; (b) improved thickening efficiency; and (c) improved ratio of the thickening efficiency to the SSI.